Atomic-resolution holography

Atomic-resolution holography with neutrons. To obtain a holographic image, one must record the interference of two coherent waves emitted by the same source. One wave must reach a detector directly while the other first scatters off of the object to be imaged. Holography using lasers has been familiar for decades, but better resolution has been achieved in recent years with electron and x-ray holography (see Physics Today, April 2001, page 21 ). Neutrons, however, because they only interact with nuclei, may offer a more versatile alternative. Last year, a group led by Laszlo Cser of the Central Research Institute for Physics in Budapest, Hungary, proposed two ways to use neutrons for holography. Soon thereafter, a group in Canada realized one of those methods—the inside-source method—using hydrogen, a strong neutron scatterer, to act as a point source of neutron waves within a sample. Now the second, the inside-detector method, has been demonstrated by Cser and his colleagues. The group placed a single crystal of lead, in which they replaced a few Pb atoms with cadmium, into a neutron beam. Because Cd absorbs neutrons 10⁶ times more readily than does Pb, the Cd acts as an internal neutron detector. The number of absorptions depends on the total neutron wave field at the Cd, including the interference between the directly arriving and previously scattered neutron waves. After absorbing a neutron, the new Cd isotope emits gamma radiation as it drops to the ground state, and those photons provide the data for the hologram. The physicists not only found the correct lattice parameter (4.93 Å) but also determined the sample’s orientation in the neutron beam. Cser believes that holography with polarized neutron beams will be valuable for studying the structure of magnetic materials. (L. Cser, et al., Phys. Rev. Lett. 89 , 175504, 2002 http://dx.doi.org/10.1103/PhysRevLett.89.175504 .)